1. Field of the Invention
The present invention relates to a process for synthesizing diamond in a film shape or a particle shape on a substrate by a vapor phase method, and an apparatus for synthesizing for same.
2. Description of the Related Art
In a diamond synthesis by a vapor phase method, a reactant gas including at least a carbonaceous source gas such as methane and the like is turned into an activated gas with plasma or the like. The activated gas is brought into contact with a surface of a substrate whose temperature is controlled at a temperature appropriate for diamond deposition, thereby depositing diamond thereon. In the diamond synthesis, non-diamond phase such as graphite and the like is generated as a by-product. Active species such as atomic hydrogen, oxygen or the like is accordingly brought into contact with the surface of the substrate simultaneously with the activated gas, thereby removing the non-diamond phase by utilizing the difference between the chemical reaction rates of the diamond and the non-diamond phase with respect to the active species.
A variety of diamond synthesis methods, for instance, a hot filament assisted chemical vapor deposition method, a microwave plasma assisted chemical vapor deposition method, an electron plasma assisted chemical vapor deposition method, a direct current electrical discharge plasma assisted chemical vapor deposition method, a radio frequency wave plasma assisted chemical vapor deposition method, a direct current arc electrical discharge plasma assisted chemical vapor deposition method, a combustion flame method and the like, have been proposed, each producing the activated gas with different means. Among them, the hot filament assisted chemical vapor deposition method and the combustion flame method are advantageous because they do not require large-scale equipment.
The above-mentioned hot filament assisted chemical vapor deposition method is an easy and convenient method. By applying the principle of the method, it is possible to enlarge a depositing area of film-shaped diamond and to increase a depositing rate thereof. In the hot filament assisted chemical vapor deposition method, an electric current flows along in a tungsten wire employed as a filament, thereby heating the tungsten wire to 2000.degree. C. or more. A reactant gas is then brought into contact with the tungsten wire, thereby producing an activated gas. However, the tungsten wire is carbonized during the course of the method, and the filament coil is deformed accordingly. As a result, the temperature distribution and the concentration distribution of the activated gas vary, and diamond deposits unevenly on a surface of a substrate. In addition, the exothermic temperature of the tungsten wire varies due to the electric resistance value change resulting from the carbonization of the tungsten wire. There has been proposed a method in which tantalum carbide is employed as the filament, however, it is hard to fulfill an enlargement of a diamond film depositing area by the proposed method.